32 U.S. NATIONAL MUSEUM BULLETIN 233 



even though its own species has been foreign to its experience until 

 then. In effect, if the young parasite is imprinted at all, it is im- 

 printed on its own species, without the need for external experiential 

 stimuli. 



In the past few decades, much work has been done on the behavior 

 of bu'ds in earliest post-hatching stages. From these investigations 

 the concept of imprinting has emerged as one of the most important 

 basic factors in the formulation of behavior — -both in the immediate 

 life of the nestling stage and even, by a supposedly ineradicable 

 residual effect, in the future adult stage. In almost all discussions of 

 this widespread phenomenon of imprinting, one thing, curiously, is 

 overlooked: some brood parasites such as cowbirds show no signs of 

 becoming imprinted on their foster-parents, certainly no signs which 

 persist for any appreciable length of time after the bird leaves the 

 nest. Since imprintmg of the young on the parent has been found 

 to be the case in an increasing list of bird species, the unusual absence 

 of it in young cowbirds deserves further examination. 



To begin with, we may take Emlen's definition (1955, p. 132) as a 

 carefully considered expression of what is meant by the concept of 

 imprinting. He considers it to be "a term applied to the rapid forma- 

 tion of stable primary stimulus-response associations or fixations 

 during early infancy. It involves the selection of a stimulus situation 

 for a newly developed and as yet unexpressed motor pattern and, 

 once formed, may affect a wide variety of motor patterns. . . . 

 Imprinting resembles conditioning but differs from it in that the 

 association formed is not a substitution but an original creation, a 

 primary association. It resembles the sign learning of Kellogg (1938), 

 but again differs from it in being independent of previously established 

 associations. It differs from trial-and-error learning in that it is an 

 additive process and not used in problem solving. . . ." We may add 

 to this the more descriptive definition given by Eibl-Eibesfeldt and 

 ICramer (1958), who write that imprmting is "an early and non- 

 selective or not discriminating innate response toward a particular 

 object or individual, and is generally characterized by the fact it 

 takes place only during a very early and very brief period in the life 

 of the animal and that it appears to have a remarkably lasting and 

 irreversible effect. The object fixation thus estabhshed by imprinting 

 persists even after long periods in which that object is kept out of 

 the animal's experience." 



Imprinting was first detected in precocial birds: they were found 

 to become fixed in their reactions by following the first moving object 

 which they encountered with any degree of intimacy; subsequently, 

 they were found to maintain a fixation upon the same object. In 

 the case of nestlings of altricial birds, imprinting is ordinarily difficult 



